1. Field of the Invention
The present invention relates to a steel cord for use in the reinforcement of rubber products such as pneumatic radial tires, particularly to a steel cord made of three steel filaments of which one filament is regularly twisted, two filaments are open-twisted and each filament is twisted in the same direction, which improves the topping processability of rubber and enhances the uniformity of the tires. The present invention also relates to a radial tire employing the same steel cord.
2. Related Art
In general, in steel cords with a so-called 1.times.3, 1.times.4 or 1.times.5 construction, for example, steel cords with a so-called 1.times.3 construction, each steel filament has the same diameter and is open-twisted. In FIG. 3-FIG. 4, conventional steel cords such as an open-twisted cord 101 and a regular-twisted cord 102 are shown. In FIG. 5, the conventional filaments 100a, 100b, 100c for open-twisting are shown in an untwisted condition.
These steel filaments are open-twisted in order to enhance the rubber penetration into the steel cord in comparison to a regular or close-twisted cord.
Further the open-twisted cord is used after being rolled once more so that the cord has an oval section(Roll Forming), or at least one of the three filaments is rolled on or through a polygonal roll before being twisted(Pre-Forming).
The open-twisted cord or the oval sectioned cord improves the rubber penetration into the cord when rubber topping. However, the low load elongation(LLE) of the open-twisted or oval sectioned cord which has its untwisted filaments having high shape maintenance property is far higher in comparison to a regular or close cord which has its untwisted filaments having low shape maintenance property. For example, in the low load range of 0.3-1.6 kgf, the elongation of the regular twisted cord is usually less than 0.08% and that of the open-twisted or oval sectioned is about 0.3%. As a results, in the open-twisted or the oval sectioned cord, a special treatment in adjusting a cord pay off tension for maintaining the open-cord shape is needed during the rubber topping procedure, and the space due to the open-twisting is reduced by causing a little more tension. Further, in the event of manufacturing a rubber sheet using the open-twisted cord, the diameter of the cord and the thickness of the topping sheet increase or vary and finally the weight of the tire increases.
To solve the above described problem, conventionally, one or two of three filaments was pre-formed by a polygonal roll before being twisted to a steel cord. This made it possible to increase the pay off tension of steel cords during the rubber topping procedure which facilitated the rubber topping procedure. However, in this case, a stress concentration was induced at the angled points by the angled filaments formed during pre-forming and so the durability of the tire was reduced.